A power transmission device using a fluid coupling is representatively constituted by an engine mounted on a vehicle, a fluid coupling, a wet friction clutch and a manual transmission, which are arranged in series. The fluid coupling provided for the power transmission device for a vehicle comprises a pump having a pump shell coupled to a crank shaft of an engine such as a diesel engine, i.e., coupled to the input shaft of the fluid coupling, and a plurality of impellers radially arranged in the pump shell; and a turbine having a turbine shell arranged facing the pump and mounted on an output shaft arranged in concentric with the input shaft, and a plurality of runners radially arranged in the turbine shell. The fluid coupling contains an operation fluid for transmitting the torque (e.g., see JP-A-2002-276694).
The fluid coupling is used for the power transmission device to absorb fluctuation in the engine rotational speed and vibration, and is applied as a start clutch for the above type of power transmission device that is used being combined to an NA diesel engine having a relatively large displacement. To cope with the exhaust gas regulations that are becoming ever stringent and the demand for better fuel efficiency, however, it is expected that the future tendency will be to use highly supercharged diesel engines (TC diesel engines) of small displacements. As compared to NA diesel engines having relatively large displacements, the TC diesel engines produce small torques when idling. Therefore, it is not desired to use a fluid coupling having a large drag torque as a start clutch. A drag torque, usually, stands for the transmission of torque of when the engine is running at an idling speed (e.g., 500 rpm). A large drag torque causes the engine to lose stability to a conspicuous degree when it is idling. Besides, the unstable revolution becomes a cause of producing abnormal vibration in the drive system. Further, a large drag torque causes the fuel efficiency to be deteriorated during the idling operation.
From the standpoint of line-up alignment of the transmission, further, it has been desired to provide a torque range which is as wide as possible by using a single kind of the transmission.
When the above-mentioned type of power transmission device is used for a TC diesel engine of a small displacement, the problem of drag torque can be tamed by using, as the start clutch, a fluid coupling of the type of variable capacity capable of decreasing the drag torque. In order to meet the demand for imparting a torque range as wide as possible by using a single kind of transmission, on the other hand, it is desired to use a torque converter as the start clutch rather than to use the fluid coupling. However, the above-type of power transmission device is combined with a manual transmission. When the torque converter having general characteristics is simply used, therefore, the torque becomes so great on the amplified output side as to exceed a permissible input limit of the transmission arousing a problem from the standpoint of strength.